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In this article we will discuss about:- 1. Meaning of Fronts 2. Creation of Fronts 3. Classification 4. Changing Weather.
Meaning of Fronts:
Front is that sloping boundary which separates two opposing air masses having contrasting characteristics in terms of air temperature, humidity, density, pressure, and wind direction. An extensive transitional zone between two converging air masses is called frontal zone or frontal surface which represents zone of discontinuity in the properties of opposing contrasting air masses.
Frontal zone is neither parallel nor vertical to ground surface; rather it is inclined at low angle. Though fronts differ from each other in terms of their location, types, and areal extent but they are characterized by the following common characteristics e.g., large differences in air temperature across a front, bending isobars, abrupt shift in wind direction, cloudiness and precipitation.
Creation of Fronts:
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When two contrasting air masses converge in deformation circulation, they spread horizontally along the axis of outflow or dialation (fig. 38.2.i). In such situation the creation of front depends on the angle between the axis of outflow and isotherms. Fronts do not form when this angle exceeds 45 degrees.
As the convergence of air continues, this angle decreases and isotherms try to become parallel to the axis of outflow and frontogenesis is activated. The steepness and intensity of fronts depends on temperature gradient. If two contrasting air masses are parallel to each other and there is no upward displacement of air, stationary front is formed (fig. 38. 2.ii). Such fronts are climatically insignificant because they are not conducive for cloud formation and precipitation.
But such situation is not very common because two contrasting and converging air masses are generally separated by sloping boundary due to deflective force (coriolis force) of the earth and cold and dense air mass pushes warm and light air mass upward (fig. 38.2.iii). It may be pointed out that fronts are not linear between two converging contrasting air masses but are zonal in character having a width of 5 to 80 kilometres.
Classification of Fronts:
Fronts are classified into four principal types on the basis of their different characteristic features e.g.:
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(1) Warm front,
(2) Cold front,
(3) Occluded front, and
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(4) Stationary front.
(1) Warm Front:
Warm front is that gently sloping frontal surface along which warm and light air becomes active and aggressive and rises slowly over cold and dense air (fig. 38.3A). The average slope of warm fronts in middle latitudes ranges between 1:100 to 1:400. The gradually rising warm air along the gently sloping warm front is cooled adiabatically, gets saturated and after condensation precipitation occurs over a relatively large area for several hours in the form of moderate to gentle precipitation.
(2) Cold Front:
Cold front is that sloping frontal surface along which cold air becomes active and aggressive and invades the warm air territory and being denser remains at the ground but forcibly uplifts the warm and light air.
Since the air motion is retarded at the ground surface due to friction while the free air above has higher velocity and hence the cold front becomes much steeper than warm front. This is why the slope of cold front varies from 1:50 to 1:100 (which means the rise of the wedge of cold air at the rate of one kilometre for every 50 to 100 kilometres).
A cold front is associated with bad weather characterized by thick clouds, heavy downpour with thunderstorms, lightning etc. Sometimes, cold frontal precipitation is also associated with snowfall and hailstorms.
(3) Occluded Front:
Occluded front is formed when cold front overtakes warm front and warm air is completely displaced from the ground surface (fig. 38.3C).
(4) Stationary Front:
Stationary front is formed when two contrasting air masses converge in such a way that they become parallel to each other and there is no ascent of air. In fact, the surface position of stationary front does not move either forward or backward.
Changing Weather Associated With Fronts:
Since fronts are formed due to convergence of two air masses of contrasting temperatures and hence contrasting weather conditions are found from north to south or south to north. Differences in terms of temperature, humidity, precipitation, cloudiness, and wind direction are experienced along different fronts e.g., warm and cold fronts.
(1) Weather Associated With Warm Fronts:
Warm air becomes active and aggressive along warm front as it invades cold air zone and thus being lighter it gradually rises over cold air and is cooled adiabatically from below. Cooling of warm air causes condensation and cloud formation followed by precipitation. If the aggressive warm air is stable and less humid, condensation occurs at great height and hence much lifting of air is required.
On the other hand, if the warm air is moist and unstable, only a slight lifting causes condensation and precipitation. The warm front precipitation is of long duration, moderate but widespread because of gentle slope of warm front. There are frequent changes in cloud types. The sequence of clouds from above downward comprises cirrus, cirro- stratus, alto-stratus and nimbo-stratus.
When warm front advances forward, the warm sector comes over the observation place. There is sudden change in weather conditions with the arrival of warm sector e.g., sudden increase in temperature and specific humidity, decrease in air pressure, disappearance of clouds, clear sky, and break in precipitation.
(2) Weather Associated With Cold Front:
Cold and dense air becomes active and aggressive along cold front wherein cold air invades warm air region and pushes it upward while it, being denser, settles downward. If cold front passes away soon, weather also becomes clear soon, otherwise if the front becomes stationary, the sky becomes overcast with cumulo-nimbus clouds provided that cold air is moist and unstable, and frontal thunderstorms are formed. Heavy precipitation occurs but is of short duration.
The consequent weather is characterized by decrease in air temperature, increase in air pressure, decrease in specific and relative humidity and change in wind direction from 45° to 180°. Precipitation is accompanied by lightning and cloud thunder. Sometimes, rainfall is associated with hailstorms. After the passage of cold front, clouds disappear, precipitation terminates and weather becomes clear and north-west cold winds set in.